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Saturn magnetises its moon Titan

Cassini imaged the many layers of Titan’s upper atmosphere in this ultraviolet image, which has been adjusted to look like true colour

(Image: NASA/JPL/SSI)

A Cassini flyby has shown Titan’s atmosphere is magnetised by Saturn. In this image, Titan is the brownish moon above the rings

(Image: NASA/JPL/Space Science Institute)

Saturn’s moon Titan is temporarily magnetised by encounters with its host planet, a recent Cassini flyby shows. The new find could help researchers understand the ultimate fate of Titan’s atmosphere. It may also help reveal whether Titan hides a salty ocean beneath its surface.

Titan’s atmosphere is a thick soup of nitrogen, as well as hydrocarbons like methane. It is thought to be very similar to the early Earth.

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But researchers don’t know how much of its atmosphere is gradually escaping into space. Like Mars, Titan seems to lack a strong intrinsic magnetic field that would protect its atmosphere from being stripped away by the solar wind – a stream of charged particles from the Sun.

It does, however, spend 95% of its time within Saturn’s magnetosphere, an oblong region of ionised gas, or plasma, surrounding the Ringed Planet. The plasma is made up of material jettisoned from the moon Enceladus, which spews icy geysers into space.

Saturn’s magnetosphere may help shield Titan from the solar wind. But it also strongly influences the magnetic fields in the moon’s atmosphere, making it difficult to discern whether Titan has its own intrinsic field.

So researchers have been hoping to study Titan when its orbit takes it outside of the magnetosphere, whose boundary flaps around in space.

Magnetic memory

After 31 close fly-bys of the moon, the Cassini spacecraft finally flew through Titan’s upper atmosphere at a time when the moon had edged out of the influence of Saturn’s magnetic field.

The encounter, which took place on 13 June 2007, showed that Titan’s atmosphere actually retains a memory of the magnetic field of the plasma that surrounds Saturn. This memory might last for as long as 3 hours.

“It’s surprising that it stays there that long,” says team member Andrew Coates of University College London. “Over long time scales, this could really help us understand how planetary atmospheres evolve.”

Titan’s temporary emergence fully exposes the moon to the solar wind. But the temporary magnetisation of the moon’s atmosphere, which produces a draped sheath of magnetic field lines, might protect it from substantial losses.

Inner ocean

The new work could help advance future efforts to discern the interior magnetic field of Titan, says Christopher Russell of the University of California, Los Angeles, who was not associated with the study.

Such an investigation might begin as early as August 2009, when Saturn will reach equinox in its orbit around the Sun, and its rings will appear edge-on to Earth observers.

The solar wind will then stream into the planet from a different angle. And that will reverse the direction of the magnetic fields induced in Titan’s atmosphere by Saturn’s magnetosphere, Russell told New Scientist.

By subtracting flyby data taken then from older data, researchers might be able to determine the strength of Titan’s intrinsic magnetic field – assuming Cassini’s extended mission lasts for another year.